1. Field of the Invention
The present invention relates to a semiconductor fabrication method and apparatus employing a rapid thermal annealing (RTA) process. More specifically, the present invention relates to an RTA system and method having improved monitoring and control of annealing and device activation.
2. Description of the Related Art
Rapid thermal annealing (RTA) is a semiconductor fabrication technique using short-time, high temperature processing to avoid unwanted dopant diffusion that would otherwise occur at the high processing temperatures of 900.degree. C. to 1000.degree. C. or greater that are used to dissolve extended defects in silicon (Si) and gallium arsenide (GaAs). The duration of an RTA process ranges from seconds to a few minutes so that semiconductor substrates are subjected to high temperatures only long enough to attain a desired process effect but not so long that a large degree of dopant diffusion takes place. RTA is typically performed in specially-designed systems rather than conventional furnaces or reactors which include susceptors, wafer boats and reactor walls having a large thermal mass which prevents performance of rapid thermal cycling. Early RTA processes used lasers as an energy source, allowing a high degree of heating to occur within fractions of a microsecond without significant thermal diffusion. Unfortunately, the wafer surfaces had to be scanned by small spot-size laser beams, causing lateral thermal gradients and wafer warping.
Subsequently, large-area incoherent energy sources were developed to overcome these limitations. These energy sources emit radiant light, which then heats the wafers, allowing very rapid and uniform heating and cooling. RTA systems have been developed in which wafers are thermally isolated so that radiant, not conductive, heating and cooling predominates. Temperature uniformity is a primary design consideration in these systems so that thermal gradients, which cause slip and warpage, are avoided. RTA systems use various heat sources including arc lamps, tungsten-halogen lamps, and resistively-heated slotted graphite sheets.
Thermal processing, such as RTA, is used in many steps of device fabrication in silicon wafers. Thermal processing is used to remove defects in silicon substrate which result from processing steps having a destructive effect. One example of a processing step having a destructive effect is ion implantation, the introduction of selected impurity dopants into a substrate using high-voltage ion bombardment to modify electronic properties of the substrate.
Thermal processing does more than simply repair implantation damage. Thermal processing is also highly useful for electrically activating the implanted impurity atoms. Upon implantation most implanted impurities do not occupy substitutional sites in the substrate.
One technique for determining the effectiveness of thermal activation of impurities is to perform Hall effect measurements. A Hall effect measurement is difficult to make in a process wafer, involving the placing of a current-carrying conductor in a magnetic field having a direction perpendicular to the direction of the current and the wafer surface.
Thermal activation is alternatively checked more simply by measuring the sheet resistance R.sub.s of the substrate wafer.
A problem with Hall effect measurements, sheet resistance measurements, and other electrical measurements are that the measurements are very difficult to make while annealing is taking place. In some cases, voltages are generated due to junctions in the wafers.
Because electrical measurements are generally not available during the annealing process, annealing is conventionally monitored by measuring temperature of a substrate wafer and activation is presumed to occur at a particular temperature or when a defined temperature is applied for a designated time duration.
However, several difficulties arise in achieving temperature uniformity. First, to raise the temperature of a semiconductor wafer of course requires heating of the slide carriers and insertion equipment for handling the wafer. The large thermal mass of slide carriers and insertion equipment extend the process times to obtain reproducible results. Significant changes in the doping profile of the wafer can occur over this time, causing difficulty in forming a desired structure in the substrate. For example, the precise alignment of shallow junctions becomes difficult to control when the temperature is not controllable.
A second problem is that dopants such as arsenic can be lost through preferential evaporation effects. In GaAs, arsenic loss is severe with considerable deterioration of the semiconductor material unless the semiconductor is appropriately capped.
Temperature uniformity is typically tested by measuring the emissivity of a semiconductor wafer using an optical pyrometer, such as an infrared pyrometer. Emissivity is defined as the ratio of power per unit area radiated from a surface to the power radiated by a black body at the same temperature when radiation is produced by the thermal excitation or agitation of atoms or molecules. When a semiconductor wafer is heated, such as occurs in rapid thermal annealing, the temperature of the wafer is raised and the increase in temperature is detectable by an optical signal with a characteristic spectrum that is indicative of the wafer temperature. Ideally a measurement of emissivity quantifies the characteristic spectrum.
Thus, the conventional usage of an infrared pyrometer ignores emissivity in other regions of the spectrum, tantamount to an assumption that emissivity occurs at a constant level across a broad spectrum and that the infrared regions is highly representative of the emissivity of the broad spectrum. However, these assumptions are erroneous.
As a semiconductor wafer is illuminated, the wafer absorbs part of the energy and reflects part of the energy. The relative amount of energy reflected and absorbed is highly dependent on the type of films on the wafer, which may be highly variable from wafer to wafer. The relative amount of energy that is reflected and absorbed is highly position-dependent in the wafer. The wafer surface generally includes various oxides, polysilicon, deposited oxides and the like, generally having variable thicknesses and types. Differences in both the type of material and the thickness of the material on the semiconductor wafer relate to variability in the absorption and reflectivity of local areas of the wafer, causing variations in emissivity at different regions of the semiconductor wafer. For example, absorption of radiant heat by the semiconductor wafer is related to the free carrier concentration so that the heating rate for heavily doped material is more rapid than for semiconductor wafers with less doping.
Nulls occasionally occur in which substantially no energy is reflected and thereby detected by the infrared pyrometer. In particular, the various types of deposits and deposition thicknesses act as a quarter-wave plate in which energy is absorbed in a material of a particular type and thickness which is coincident with the effective wavelength of the pyrometer so that a quarter-wave path difference with a relative phase shift of 90.degree. occurs between ordinary and extraordinary waves. Thus, substantially all of the energy at the effective wavelength of the pyrometer is absorbed in the material and very little is reflected. The pyrometer badly misjudges the temperature of the wafer in these regions, measuring a temperature that is much lower than the actual temperature.
The temperature measurement system is typically used in a feedback control system which responds to the detected low temperature by increasing the intensity of the heating lamps or extending the duration of annealing. The increase in RTA processing damages or destroys the semiconductor wafer in process.
Present day rapid thermal anneal systems typically address the problems of emissivity measurement variations and temperature measurement inaccuracies by attempting to construct an ideal RTA chamber, specifically an RTA chamber which is most equivalent to a black body radiator so that the only energy absorbing component in the chamber is the semiconductor wafer. However, even with an ideal RTA chamber, absorption by the semiconductor wafer introduces variability in temperature measurement that may not be compensated.
Another problem with monitoring annealing and activation using a temperature measurement is that the absorption of energy by the wafer depends not only on temperature but also on many other factors including dopant concentration. As impurities become activated, the absorption of energy is modified.
Furthermore, variability in activation temperature is inherent throughout the substrate wafer due to constructive and destructive interference in systems using optical temperature measurements such as optical pyrometers. Commonly rapid thermal anneal chambers supply heat using lamps on both the frontside and the backside of a wafer. The backside of a wafer typically has multiple films with different optical properties. The variability in optical properties leads to constructive and destructive interference which makes a temperature measurement highly variable.
What is needed is a method and system for monitoring and accurately controlling activation of impurities or dopants in a rapid thermal anneal system. What is further needed is a method and apparatus for determining the conditions of annealing in a substrate wafer at which an impurity such as boron and phosphorus is annealed in a silicon lattice using an optical monitoring technique.